The present invention is directed to an audio amplifier and, more particularly, to an audio amplifier employing an autotransformer to effect impedance matching between the amplifier power transistor output and a load, such as a loud speaker system.
Audio amplifiers are commonly designed to accommodate any of several different loads having different impedances. Typically, an audio amplifier will be provided with output terminals for driving speaker systems having 2, 4 or 8 ohm impedances. In order to provide impedance matching between the amplifier circuit and the loud speaker system, conventional audio amplifier circuits employ output transformers which have a primary winding driven by the power output stage of the amplifier and an electrically separate secondary winding having a plurality of taps to provide plural outputs at different impedance levels. Such conventional output transformers are capable of providing high fidelity output signals with a minimum of distortion throughout the audio range. However, these transformers are, of necessity, massive, typically being among the largest and heaviest components of an amplifier system.
Theoretically, the substitution of an autotransformer for a conventional primary-secondary output transformer would result in a reduction of the size, weight and expense of an audio amplifier system. With the exception of certain specialized applications, however, such arrangements have not been employed. This is due at least in part to the difficulty of obtaining an audio output signal from an autotransformer which is free of distortion throughout the audio range. Laurent et al., U.S. Pat. No. 3,478,274 and Slaats et al., Canadian Pat. No. 660,131 each disclose an amplifier circuit having an autotransformer for coupling the amplifier output signal to a speaker. Each of these circuits is intended for relatively low power operation, the amplifiers being designed for use in automobile radios. Each of these circuits require a nonconventional power output stage, this stage including a pair of transistors connected in push-pull relation and asymmetrically driven. Each of these circuits also require that the direct current power supply for the power transistors be connected through the autotransformer windings, one terminal of the direct current supply being connected to a center tap of the winding so that the supply current path for one of the power output transistors is through a first portion of the autotransformer winding and that of the other power output transistor is through at least part of the remainder of the winding. The imposition of the supply current on the winding of the autotransformer requires that the autotransformer be of greater load handling capacity than would be necessary if only the audio output signal of the amplifier was carried by the autotransformer winding. Typically, an arrangement such as that of the Laurent et al. or Slaats et al. patent requires the use of an auto transformer having a 30 percent greater power handling capacity than would be necessary if only the alternating current audio signal was handled by the autotransformer winding.
A primary object of the present invention is the provision of an audio amplifier circuit employing an autotransformer to effect impedance matching between the amplifier circuit and a load, such as a speaker system.
A further object of the present invention is the provision of an audio amplifier circuit in which the output transformer is an autotransformer, the amplifier circuit being of conventional design in all other aspects.
Another object of the present invention is the provision of an audio amplifier circuit having an autotransformer as the output transformer thereof and in which only the alternating current audio output signal is supplied to the autotransformer winding.
Yet another object of the present invention is the provision of an audio amplifier circuit having an autotransformer as the output transformer thereof and in which the autotransformer provides a plurality of outputs to accommodate different loads operating at different impedance levels.
It is also an object of the present invention to provide a novel autotransformer adapted to function as an audio amplifier output transformer and which is capable of effecting impedance matching between the amplifier and a load such as a loud speaker system with minimum distortion and degradation of the audio signals throughout the frequency of such signals.